1. Field of the Invention
The present invention relates generally to the field of telecommunications and, more specifically, to a method and apparatus for providing conference services in an expandable switching system.
2. Discussion of the Prior Art
In addition to providing two-way circuit switched connections between callers, many telecommunications systems are able to provide a variety of telecommunications services. For example, subscribers may request services such as call waiting, caller identification and call forwarding, among others. One of the more desired services, especially in commercial contexts, is conferencing. Conferencing refers to the ability of three or more callers, each utilizing a separate telephone set and often located at remote locations from each other, to participate in a single telephone call. In response to the demand for conferencing services, efforts have been made to design and incorporate conferencing systems within existing and new Private Branch Exchange (PBX) switches and public switching systems.
These conferencing systems typically interface with a conventional computer-controlled digital switching matrix within the PBX or public switching system that provides the circuit switching function. A separate bus or conference highway is usually provided between the digital switching matrix and a conference processor. In order to provide conferencing services via the conference processor, these systems typically require that the digital switching matrix interface directly with each participant in the desired conference (i.e., the conferees). That is, regardless of where the calls originate, conventional circuit switched data corresponding to each call is routed, typically through various central offices, to the single PBX or public switching system having the digital switching matrix and the conference processor.
The digital switching matrix, under the control of the computer, then switches the circuit switched data from each conferee onto the conference bus. The conference processor receives the circuit switched data and operates on that data. Typically, the conference processor executes a known conference algorithm, such as summing together the circuit switched data from each conferee. Furthermore, after summing all of the data, the conference processor typically subtracts each conferee's data from the summed total intended for that conferee, in order to minimize echo effects and improve system stability. Thus, the conference processor outputs separate conferenced data for each conferee. Each item of conferenced data is then transmitted to the digital switching matrix within the PBX or public switching system via the conference bus and the digital switching matrix switches the items of conferenced data to the corresponding conferees.
One of the drawbacks of such systems is that they rely upon the existence of a two-way circuit switched connection exchanging conventional circuit switched data between each conferee and the digital switching matrix that is performing the conferencing services. That is, these systems presuppose the existence of a PBX or public switch which receives circuit switched data from conferees interfaced directly with the PBX or public switch. Accordingly, such designs cannot be implemented in telecommunications systems that do not establish such conventional circuit switched connections.
3. Discussion of Related System Not Found in the Prior Art
To provide a telecommunications system with substantially greater flexibility than conventional PBX switches, the assignee of this application developed an expandable telecommunications system. The system, which may utilize a plurality of inter-connected switching nodes, is described in U.S. Pat. No. 5,544,163 co-owned by the assignee of this application and incorporated herein in its entirety. One or more nodes may be interfaced with the Public Switched Telephone Network (PSTN) or a private network so that the node may receive information such as voice information. Each node within the system, moreover, includes circuitry for transmitting and receiving variable-length, packetized information over the network, thereby enabling each node to receive information, such as voice information, from or transmit information to all other nodes.
One method for transmitting information between nodes, known as the Empty Send/Full Return (ESFR) method, is for each node to formulate and transmit one or more packets which has an "empty" payload. The empty packets are received first by an adjacent node on the network which determines whether it has any information to send to the node from which the packet originated. If so, the adjacent node inserts that information into the payload of the packet before the packet passes to the next adjacent node on the inter-nodal network. If the next adjacent node has no information for the packet-originating node the packet simply proceeds to the next adjacent node. This process is repeated at each node until the packet traverses the entire network and returns to the originating node. At this point, the packet will have a "full" payload of information from the other nodes. The originating node then captures the information from the payload. Each node similarly transmits "empty" packets across the network which return filled with information from other nodes intended for the packet-originating node.
Alternatively, a node may formulate and transmit a packet having a payload that is "full" of information destined for other nodes (or for itself). This is known as the Full Send/Empty Return (FSER) method. With this method, the node inserts information intended for other nodes or for itself into a packet and transmits that packet across the network. Each packet is initially received by an adjacent node which determines whether any information in the payload of the packet is addressed to that adjacent node. If so, the information is captured from the payload by the adjacent node before the packet passes to the next adjacent node. The next adjacent node similarly captures any information addressed to it and this process is repeated until each node in the network has captured the information, if any, intended for it from the payload of the packet. Similarly, each node transmits a packet having a full payload, thereby allowing each node access to the information originated by each other node. It should be understood that either process, ESFR or FSER, or a combination of such processes may be utilized to exchange voice information between two callers each of whom is interfaced with a different node, thereby establishing a "connection" between the two callers.
Despite their utility in conventional PBX switches, the conferencing techniques described above are not readily transferable to a system, such as that just described, in which a plurality of nodes transmit variable-length packets over a high-speed network. More specifically, it is neither possible nor practicable to establish a conventional circuit switched connection between a plurality of callers and a single node. Accordingly, new means of providing conferencing services in such systems must be developed.